Moved dicache entry removal to a_Dicache_cleanup()

This allows keeping the decompressed image entry until a_Dicache_cleanup()
removes it. With the added SurvCleanup variable that controls how many
cleanup passes the entry is kept, it is now possible to tune the dicache.

For instance:
 SurvCleanup=0 same as without patch (negligible difference).
 SurvCleanup=1 allows Back and Forward reuse entries for adjacent pages.
 SurvCleanup=2 all the above, plus one more pass of lifetime.

The default is SurvCleanup=3 (experimental).
Example: If you read a newspaper with lots of images, this caching allows
to click an article, read it, and go back *quickly* using the same tab.

It is experimental because there may be other simple approaches that serve
well/better these kind of usage patterns.

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